Electric fire detector



Nov. 8, 1949 T. M. DAHM ETAL Y 2,487,526

ELECTRIC FIRE DETECTOR Filed April 14, 1947 INVENTORS THOMAS M. DAHM MELVIN F. GEORGE JR. BY

Patented Nov. 8, 1949 ELECTRIC FIRE DETECTOR Thomas M. Dahm, Pasadena,

George, Jr., Los Angeles, Lockheed Aircraft Corporation,

and Melvin F. Calif., assignors to Burbank, Calif.

Application April 14, 1947, Serial No. 741,232

15 Claims.

This invention relates to temperature responsive devices and relates more particularly to devices of this character suitable for use as re detectors and operable to control electrical circuits of ilre warning and ilre extinguishing systems.

Fire detectors have been suggested which embody a tube constituting one electrode of a circuit, a wire or conductor enclosed in the tube and forming the second electrode, the two tubes be ing spaced by a material which has a high electrical resistance at normal temperatures but which becomes conductive to complete a normally open circuit when the device is subjected to elevated temperatures. Such prior proposals have definite shortcomings which limit their successful practical applications. For example, means for sealing the outer tube are either absent or ineffective with the result that the material, usually a salt or salt mixture which becomes conductive at elevated temperatures, is adversely aiected by atmospheric conditions, condensation of moisture, etc. rendering the device unreliable. Furthermore, the salt or material arranged between the electrodes of prior proposals either deteriorates with age and repeated use, or loses its sensitivity so that the temperature range in which the detector becomes operative is uncertain. These and other considerations tend to make the earlier detectors of this general class unreliable and therefore of little or no practical value.

It is a general object of this invention to provide an effective re detector of the class above referred to that is positive and dependable in operation and that is substantially unaffected by aging at elevated temperatures or repeated usage. The device of this invention retains a given or selected range of temperature actuation following repeat oprations and is unaffected by age. atmospheric conditions, etc. Accordingly, the device is adapted for installations where rekindling of a re may occur and where the device is subject to non-critical temperature fluctuations, atmospheric variations, etc.

Another object of the invention is to provide a iire detector of the class mentioned that may be embodied in a relatively small, compact unit that is permanently hermetically sealed and suitable for spot installation, that is for installation in selected locations where fires are apt to occur 0r Where special fire protection precautions are indicated.

Another object of the invention is to provide a ilre detector of the character mentioned in which the inner electrode is in the form of a thin-walled metal tube which has a low heat capacity per unit area so as to rapidly rise in temperature upon receiving only a small amount of heat. This materially accelerates the action of the device. The salt which occupies the space between the two electrodes cannot melt through to close the electrical circuit until the surface temperature of the inner electrode has been raised to the melting point of the salt. Where the inner electrode is a solid rod or wire, as in prior devices, the action is slow because the electrode has a high heat capacity per unit area. Inthe present invention the outer electrode is a thin-wa-lled tube to have a minimum temperature diierential between its inner and outer surfaces when heated by a flame and to have a low heat capacity per unit area to transmit a maximum of heat to the salt. The thin-walled construction of the two electrodes increases the sensitivity of the detector to flame and elevated temperatures and is an important factor in increasing the rate of response of the device.

Another object of this invention is to provide a lire detector in which the salt is arranged or contained in a manner to materially increase the rate at which the heat reaches the inner electrode and therefore to increase the rate at which the salt will become electrically conductive to render the detector active. The salt is a poor thermal conductor and has considerable heat of fusion so that its presence between the electrodes reduces the rate at which the heat reaches the inner electrode. In the present invention, the salt is supported as thin layers on the multitudinous strands of a woven sleeve to normally constitute an effective dielectric between the two electrodes and yet be in a state or condition to offer a minimum of resistance to the transmission of heat between the outer and inner tubular electrodes.

Other objects and features of the invention will become apparent from the following detailed de'- scription of typical preferred forms throughout which description reference is had to the accompanying drawings, in which:

Figure 1 is a longitudinal sectional view of a detector of the present invention;

Figure 2 is an enlarged transverse sectional view taken as indicated by line 2-2 on Figure 1;

Figure 3 is a perspective view of the inner electrode assembly removed from the outer electrode;

Figure 4 is a longitudinal sectional view of another form of detector of the invention; and

Figure 5 is an enlarged transverse sectional view taken as indicated by line 5--5 on Figure 4.

The uru't illustrated in Figures 1, 2 and 3, in-

55 cludes a mounting flange or plate l0 and an outer tube II secured thereto. The plate I may be of any required or selected shape and construction. In the drawings, the plate I0 is a simple at member provided with a central opening I2 and marginal openings I3, the latter being adaptf ed to receive attaching screws or the like to facilitate the mounting of the detector. The tube II is an elongate part secured to the ilange or plate I0 to project therefrom. The inner end of the tube II is engaged in the opening I2 and is fixed to the plate I0 by welding or by a solder having a high melting point. The outer end of the tube I I is closed and sealed by a cap I4 welded, brazed, or soldered in place. The tube II constitutes one of the electrodes and is.;l therefore, formed of metal. We have found it desirable to construct both the tube II and the ange I0 of stainless steel. In accordance with the inven-` tion, the tube II has a thin wall, say below .007" thick. For example, the tube II may be formed of stainless steel tubing having a Wall thickness of .005".

The inner electrode 0f the detector is in 'the form of a metal tube I5 arranged in the outer electrode I I in coaxial relation thereto. The tube or electrode I5 extends from the plane of the ange I0 to adjacent the cap I4 and has a diameter smaller than the inside diameter of the outer tube II, leaving an annular space between the two concentric electrodes. It is a feature of the invention that the tubular inner electrode I5 has a small Wall thickness. The wall thickness of the tube I5 is preferably less than .007" and in practice may be .005". The head I6 of a terminal stud I'I is secured in I5 by silver solder, or the like. The stud I'I is employed as a post or terminal for one side or lead I8 of an electrical fire warning or re extnguishing circuit and assists in securing the tube sealing means to be subsequently described.

The annular space vbetween the above described tubular electrodes II and I5 contains a tubular sleeve 20 of glass threads or other dielectric or insulating brous material. This sleeve 20 serves as a carrier for the salt or salt mixture 2I indicated by the dots in the drawings. The impregnated sleeve rather closely fits around the inner electrode I5 and rather closely engages in the outer electrode II. The outer end portion of the sleeve 20 is turned or wrapped around into the outer end of the inner electrode I5, and a plug 22 of glass or ceramic material is entered in the turned back part of the woven sleeve to retain it in position. The plug 22 is in turn engaged against the cap I4. While we have found this manner of securing the sleeve 20 tothe inner electrode to be effective, it is to be understood that other appropriate means may be used for this purpose. The other end of the sleeve 20 occurs at or adjacent the corresponding ends of the two electrodes and a washer 23 is arranged on the stud to engage the end of the sleeve. Thewasher 23 may be metal having a covering of vitreous enamel or other dielectric material or may be aluminum with a heavy anodic coating. In any case the washer 23 must not electrically connect the electrode tubes II and I5.

The above mentioned salt or salt mixture 2| is impregnated in the woven screen 20 to form a spacer of high electrical resistance between the two electrodes II and I5, but upon approaching the fusion temperature the electrical resistance of the salt rapidly lessens and the salt then prothe inner end of the tube vides a path for the electricity to complete the circuit between the conductor I8 and the inner electrode I5 on the one hand and the outer electrode II and a conductor 24 on the other hand. A re warning device 25, a fire extinguishing device, or the like, is connected in the lead 24 to be energized when lthis occurs. The salt 2| is deposited on the surfaces of the sleeve 20 in relatively thin layers to absorb a minimum amount of heat and therefore increase the rate at which heat is conducted to the inner electrode I5.

Furthermore, the .thin layers or coatings of the salt reduce the thermal insulating effect of the salt to a minimum. It will be observed that the sleeve 20 of glass fibers or the like, in addition to forming a carrier for the thin layers of salt constitutes a dependable mechanical insulator for preventing metallic short circuiting of the electrodes I I and I5, even at a time when the salt 2| is fused. Such a short circuiting might persist after cooling land solidification of the salt.

The particular salt or salt mixture employed depends to a large degree upon the intended temperature at which the detector is to operate. For installations such as the engine nacelles of aircraft, where it is desired to have the lre detector operate at a temperature of between 425 F. and 430 F., we prefer to employ a salt mixture of 50% sodium nitrate and 50% sodium nitrite, all percentages referred to herein being mole percentages. We have found this to besuperior to a sodium nitrate and potassium nitrate eutectic which melts sharply at approximately 424 F. The sodium nitrate and potassium nitrate mixture has a considerably reduced operating temperature after repeated operation, probably due to the fact that the system becomes a complex mixture of NaNOa, KNOa, NaNOz, and KNOz.v In order to elminate this, the system NaNOs, NaNOz is chosen wherein only salts of a single akali metal are used and the chance of decompos'tion of the NO3 to NO2 is greatly decreased due to the equilibrium effect. Even though some decomposition of the nitrate ion to the nitrite ion ddes occur, the salt system remains binary. The conductivity point or that point at which the initial melting of the salt mixture occurs remains unchanged due to the presence of the eutectic. The preferred mixture melts at 428 F. although there may be slight variations in the melting point after repeated operations.v Various lead and silver halides may be used but due to electrolysis effects they form products which render them unsuited where metallic containers or electrodes are employed. On the other hand, alkali metal and alkaline earth metal ions are not readily plated out from their salts and do not corrode the stainless steel electrodes II and I5. Accordingly alkali metal and alkaline earth metal salts are preferred.

It is contemplated that the mechanical features of the detectors may be made standard and that the salts may be varied or changed to alter the operative temperatures of the units for given installations. For example, where the operative temperature is to be 600 F. the sleeve 20 maybe provided with a deposit or covering of 20% potassium nitrate and potassium nitrite and when the operative temperature is to be 480 F. lithium nitrate may be used. Again, where the operative temperature is to be 525 F. 100% sodium nitrate may be provided on the sleeve and where the device is to operate at a lower' temperature of about 350 F. 62% thallium nitrate and 28% potassium nitrate may be used.

The electrode assembly is hermetically sealed to assure consistent and reliable operation of the detector. The sealing means includes a synthetic rubber washer 28 engaged against the top of the washer 23. A tubular metal cage 21 is brazed or otherwise xed to the flange I and `the washer 26 is pressed into the cage to bear against the washer 23. An outer washer 28 of metal or the like is arranged on the outer side of the synthetic washer 26 and a nut 29 is threaded on the stud I1 to squeeze or force the washer 26 into tight effective sealing engagement with the stud, the washer 23 and the cage 21. A cap 30 of synthetic rubber is vulcanized on the cage 21, stud I1 and washer assembly to provide the hermetic seal.

In assembling the device the impregnated woven sleeve 20 is slipped over the inner electrode I and is then inserted in the outer electrode II. It is preferred to heat the parts to the meltingpoint of the salt before performing these operations to provide lubrication and to protect the sleeve against injury. The assembly is then dried thoroughly until it operates at between 410 F. and 420-F. whereupon the above described washer arrangement is assembled to prevent subsequent moisture absorption by the salt. The nal step in the assembly is the vulcanizing on of the cap to provide the hermetic seal.

Figures 3 and 5 of the drawings illustrate a detector of the invention which is sealed by a glass closure and a synthetic rubber cap. In this construction the flange III, inner electrode I5, sleeve 20 and electrical circuit may be the same as described above and corresponding reference numerals are applied to these parts. electrode is substantially the same as in the previously described form of the invention, however the outer end of the electrode is formed over a glass block 4I and its end is closed by welding at 42. A tubular cage or shell 43 is secured to the flange I0 in coaxial relation to the electrodes and is closed by a body or closure 44 of glass applied to the end of the shell while in a molten state. A wire 45 is embedded or cast in the glass closure and extends into the inner electrode I5. The outer end of the wire 45 is brazed or soldered to the interior of the electrode I5 and the opposite end of the wire is secured conductively to a threaded terminal stud 46. It is preferred to allow coiled slack in the wire 45 to facilitate the attachment of the wire to the electrode I5, either subsequent to or prior to formation of closure 44 and prior to arrang ing the bloclrv 4I in place. Following the assemblina,y drying and initial glass sealing of the unit, a cap 41 of synthetic rubber or the like, is vulcanized on the shell 43 and glass closure 44 to constitute the final hermetic seal.

It is believed that the operation of the detectors will be readily understood from the foregoing detailed description. Referring now to the embodiment of the invention illustrated in Figures 1 to 3, the flange II) may be employed as a means for mounting the detector in the selected position. The detector, being small and compact, may be readily mounted at or adjacent the point where fire is likely to occur or where re protection is important. Under normal conditions, that is when the temperature is well below the fusion point of the salt 2|, the electrical circuit of the nre warning or re extinguishing device 25 remains open owing to the fact that the salt 2l constitutes an insulator or body of high electrical resistance between the two electrodes II and I5. However, when the temperature of the electrodes The outer approaches the melting point of the salt, for example when the nre detector is subjected to a flame,l the thin deposit of salt on the sleeve 20 rapidly reaches a temperature where it constitutes a good electrical conductor between the two electrodes. The electrical conductivity of the salt rather rapidly increases before and after the melting point is attained. As described above, the thin-walled construction of the tubes II and I5, and the impregnation of the fiber glass sleeve 20, contribute materially to the rapid action of the detector when subjected to the elevated temperature. When the circuit is completed through the salt, the device 25 is operated. Subsequently, when the fire detector has cooled, the salt` 2| returns to its solid condition and again forms a body or sheath of high electrical resistance between the electrodes II and I5. The detector is capable of numerous repeat operations; for example it will again close the electrical circuit to the device 25 in the event the original fire rekindles or a second nre subsequently develops.

Having described only typical forms of the invention we do not wish to be limited to the specific details herein set forth, but wish to reserve to ourselves any variations or modifications that may appear to those skilled in the art and fall within the scope of the following claims.

We claim:

l. A device of the class described comprising a tubular outer electrode, a tubular thin-walled inner electrode of low heat capacity per unit area enclosed by and spaced from the inner wall o f the outer electrode, a fusible salt composition in the space between the inner electrode and raid inner wall that is non-conductive when cold and that becomes conductive when it approaches the fusion temperature, and means hermetically sealing the ends of the outer electrode.

2. A device of the class described comprising a tubular outer electrode, a tubular inner electrode enclosed by and spaced from the inner wall of the outer electrode, a fusible salt composition in the space between the inner electrode and said inner wall that is non-conductive at normal ternneratures and that becomes conductive when its temperature approaches the fusion temperature, means closing one end of the outer electrode, a terminal on the inner electrode extending beyond the other end of the outer electrode, and a hermetic seal for said other end of the outer electrode.

3. A flame and temperature sensitive device comprising a tubular outer electrode, a tubular inner electrode within the outer electrode, there being an annular space between the inner electrode and the internal wall of the outer electrode, a tubular woven sleeve of non-conductive material occupying said annular space and surrounding the inner electrode. and a salt impregnated in said woven sleeve that remains non-conductive until its temperature approaches the fusion temperature whereupon it becomes conductive to provide an electrical path between said electrodes.

4. A flame and temperature sensitive device comprising a thin walled tubular outer electrode, a thin walled tubular inner electrode within the outer electrode, there being an annular space between the inner electrode and the internal wall of the outer electrode, a tubular woven sleeve of non-conductive material arranged in said annular space to surround the inner electrode, and a salt impregnated in said woven sleeve that remains non-conductive until its temperature approaches the temperature of fusion whereupon it becomes conductive to provide an electrical path between said electrodes.

5. A flame and temperature sensitive device comprising a tubular outer electrode, a tubular inner electrode within the outer electrode, `there being an annular space between the inner electrode and the internal wall of the outer electrode, a tubular woven sleeve of non-conductive material arranged in said annular space to sur round the inner electrode, means for closing and sealing one end of the outer electrode, a terminal stud on the inner electrode extending beyond the other end of the outer electrode, an hermetic seal means closing said other end of the outer electrode and sealing around said stud, and a salt impregnated in said woven sleeve that is non conductive when cold and that is conductive to provide an electrical path between said electrodes when its temperature approaches the temperature of fusion.

6. A device of the character described com prising a closed hollow outer electrode, a thin walled tubular inner electrode within the outer electrode, the inner electrode being spaced from the inner wall of the outer electrode to leave an annular space, a porous tubular woven sleeve of non-conductive material arranged in said space to surround the inner electrode, and fusible salt d iiused through the pores of said sleeve, said salt being non-conductive until it approaches the temperature of fusion whereupon it becomes conductive to form an electrical path between said electrodes.

'7. A device of the character described compris-I ing a closed hollow outer electrode, a thin walled tubular inner electrode within the outer electrode, the inner electrode being spaced from the inner wall of the outer electrode to leave an annular space, a porous tubular woven sleeve of non-coin ductive material in said space, and a mixture of sodium nitrate and sodium nitrite dliused through the pores of said sleeve, said mixture being non-conductive until it approaches the ternperature of fusion whereupon it becomes conductive to form an electrical path between said electrodes.

8. In a device of the class described, the combination of two tubular electrodes in spaced substantially concentric relation, a sleeve of woven glass fibers arranged in the space between the electrodes, and a salt composition impregnated in said sleeve, said salt composition being nonconductive until it approaches the temperature of fusion whereupon it becomes conductive to provide an electrical path between said electrodes.

9. In a device of the class described, an outer tubular electrode, an inner electrode withfnthe outer electrode and spaced from the wall thereof to leave an annular space, a porous sleeve of woven glass fibers in said space, and a mixture of sodium 'nitrate and sodium nitrte diffused through the pores of said sleeve, said mixture being non-conductive when cold and being conductive when it approaches the fusion temperature.

10. A device of the character described com` prising a tubular outer electrode having an end wall closing one of its ends, a tubular inner electrode within the outer electrode and spaced from the wall thereof to leave an annular space, a woven sleeve of dielectric material in said space, a salt composition impregnated in said sleeve and non-conductive when cold but conductive to provide an electrical path between the electrodes when it approaches the fusion temperature, a

terminal engaged in the inner electrode and extending beyond the other end of the outer electrode, a washer assembly on the terminal for seal ing said other end to the outer electrode, and a nut threaded on the terminal to actuate said assembly.

1l. A device of the character described comprising a tubular outer electrode having an end wail closing one of its ends, a tubular inner electrode within the outer electrode and spaced from the wall thereof to leave an annular space, a woven sleeve of dielectric material in said space, a salt composition impregnated in said sleeve that is non-conductive when cold but conductive to provide an electrical path between the electrodes when it approaches the fusion temperature, a

terminal secured to the inner electrode and extending beyond the other end of the outer electrode, a washer assembly on the terminal for sealing said other end of the outer electrode, and a sealing cap vulcanized on the terminal and said assembly.

l2. A device of the character described comprising a tubular outer electrode having an end wall closing one of its ends, a tubular inner electrode within the outer electrode and spaced from the wall thereof to leave an annular space, a woven sleeve of dielectric material in said space, an end portion of the sleeve being wrapped about an end of the inner electrode and engaged in the interior thereof; a plug engaged in said portion of the sleeve to secure the sleeve to the inner electrode and engaging said end wall of the outer electrode, a salt composition impregnated in said sleeve that is non-conductive when cold but conductive to provide an electrical path between the electrodes when it approaches the fusion temperature, a terminal secured to the inner electrode and extending from the other end of the outer electrode, a washer assembly on the terminal for sealing said other end of the outer electrode, and a nut threaded on the terminal to actuate said assembly.

13. A device of the class described including a tubular outer electrode, means closing one end of the outer electrode, a tubular inner electrode spaced from `the wall of the outer electrode to leave an annular space, a porous woven sleeve of dielectric material in said space, a salt composition diffused through the pores of the sleeve that is non-conductive until it approaches the fusion temperature whereupon it becomes conductive to form an electrical path between the electrodes, a glass closure sealing the other end of the outer electrode, and a wire embedded in and extending through the glass closure and connected with the inner electrode.

14. In a device of the class described, a tubular outer electrode, a thin walled tubular inner electrode within and spaced from the wall of the outer electrode to leave an annular space, a porous sleeve of woven glass bers in said space, and a mixture of approximately 50% sodium nitrate-and approximately 50% sodium nitrite diiused through the pores of said sleeve.

15. A device of the character described comprising a closed hollow outer electrode, a thin walled tubular inner electrode within the outer electrode, the inner electrode being spaced from the inner wall of the outer electrode to leave an annular space, a relatively thin tubular woven sleeve of non-conductive material in said space, and a thin layer of salt on said sleeve that is nonconductive, until the temperature of fusion o f 9 the salt is approached whereupon it becomes conductive to provide an electrical path between the Number electrodes. 2 THOMAS M. DAHM. 2,341,219 MELVIN F. GEORGE, JR. 2,413,125

REFERENCES CITED Number The following references are of record in the 537,155 le of this patent: 557,765

10 UNITED STATES PATENTS Name Date Kernen Apr. 20, 1943 Jones Feb. 8, 1944 Walbridge Dec. 24, 1946 FOREIGN PATENTS Country Date Great Britain June l1, 1941 Great Britain Dec. 3, 1943 

